I made a blackberry soda using frozen berries, carbed with yeast. I went about it just like any fruit soda... simmer the berries for about 15 minutes, strain the pulp and seeds, add sugar to taste, cool, pitch the yeast, bottle until firm, chill, enjoy. The berries had good flavor, but not much body or bite, so I added a bit of fresh ginger and a pinch of mint. I usually make about 70 oz. total, which ends up in one 20 oz plastic bottle (to check carbonation) and 3 16 oz. glass bottles. Don't worry too much about a recipe, just go by taste. If your mix tastes good luke-warm, it should taste fabulous when carbonated and chilled.